1. Field of the Invention
This invention relates to electroacoustic transducers in general, and to ultrasonic level indicators and controls incorporating such transducers in particular. Though it is especially useful for ultrasonic frequency applications, it also has beneficial uses in sonic frequency applications as well.
2. Description of the Prior Art
Electroacoustic transducers are well-known devices for converting electrical signals into sound waves and sound waves into electrical signals.
Such transducers used with conventional ultrasonic level controls act as both loudspeakers and microphones. Acting as a loudspeaker, the transducer emits a burst of high frequency sound energy which is above the range of human hearing. This sound wave travels down into a container, such as for example a storage tank, where it is reflected from the surface of the substance whose level is being measured. The reflected wave or "echo" is then picked up by the same transducer, now acting as a microphone. Thus, the transducer performs both the sound transmitting and receiving functions.
The elapsed time between the sending of the transmitted signal and the arrival of the received signal is proportional to the distance between the transducer and the reflecting surface. Measurement of this time lag can thus be translated electronically into measurement of the level. The signal thus obtained may be used for control purposes and is usually displayed on a digital or analog indicator and/or used to control input or exit valves to the tank.
A major advantage of ultrasonic level control systems is the contact-free nature of their operation. This permits the measurement of sticky or gummy materials, of food stuffs where contact is unhygienic and of highly abrasive or corrosive materials which would act to degrade any level indicator that comes into contact with the measured substance.
Two types of transducers are widely used in ultrasonic level control applications: magnetic transducers and piezoelectric transducers (commonly called crystal transducers).
Magnetic transducers operate much like conventional loudspeakers. They have similar construction to a high-fidelity, high frequency tweeter-type loudspeaker. However, custom designed magnetic transducers adapted for ultrasonic level control use are very expensive, costing up to several hundred dollars apiece, and have very high power requirements.
Piezoelectric transducers, on the other hand, are relatively inexpensive, generally costing only about one-eighth to one-quarter as much as their magnetic counterparts. Some basic crystal transducers are available at a cost of under ten dollars. Crystal transducers have several disadvantages, however, in their use as ultrasonic level control devices.
For level control operations in most chemical process applications, detection ranges on the order of 1.5 feet to 20 feet, or more, are desired. Because of these range requirements, the present state of the art piezoelectric transducers used in ultrasonic level control applications are typically the larger and more expensive elements, that are constructed of such materials as barium titanate and lead zirconate titanate. The physical size of these elements is typically about 2 inches or more in diameter and a 1/4 inch or more in thickness. Such sizes are required to deliver the large amount of power needed to achieve good maximum range detection. Very high power, on the order of up to 1,000 watts at a 1 percent duty cycle (time on various time off) is conventionally used to obtain long range operation (above 15 feet.).
Piezoelectric transducers that satisfactorily send and receive level control information over long distances do not, however, perform satisfactorily in low temperature environments. The reason for this is that piezoelectric elements which are large enough to achieve these long ranges tend to become brittle and crack when performing in low temperatures. Typical operating temperature range figures for a magnetic transducer unit are -40.degree. F. to +200.degree. F.; the typical range for a conventional crystal unit is about +32.degree. F. to +200.degree. F. Conventional crystal devices are thus not suitable for measuring levels in outdoor storage and transfer tanks in cold climates as in Minnesota or Alaska.
On the other hand, the relatively inexpensive small transducer elements, sized on the order of 1/4 inch in diameter and 10 mils thick, which operate at low power and whose performance is not degraded by low temperatures, have poor range detection performance characteristics and do not possess the long range capabilities of large crystal units. The maximum range of typical small transducer units is about 1 to 2 feet.
Both large and small crystal units have disadvantages which affect their ability to have suitable minimum range detection characteristics. One such disadvantage is the existance of undesirable side lobes in the directional radiation pattern of the transducer. Such side lobes represent signals that are emitted in unwanted directions and cause false echoes, such as by being reflected from the sides of storage tanks instead of from the surface of the substance whose level is being measured. For use as a proximity indicator, the transducer sound dispersion pattern should be as restricted as possible, and preferably should be in a searchlight beam type of pattern.
Another disadvantage that limits the minimum distance for crystal transducer detection is the phenomenon of "ringing". Ringing is the tendency of an excited crystal to sustain oscillation in its natural resonant frequency mode after the excitation signal has been removed. Ringing is especially troublesome in ultrasonic level control operations because the piezoelectric element is usually driven at its resonant frequency. Ringing causes errors in level detection at short distances because at such distances, the signal has a much shorter travel time. Thus, the transducer may still be ringing when the echo signal is received, which makes it impossible to distinguish between the ringing signal and the echo signal. For short range distance detection, reflection of the backwave, as from the rear of the transducer housing, causes problems similar to ringing.